There is tremendous demand for chemical and bioanalytical devices that enable high throughput and performance assays in small point-of-care or point-of-analysis devices. Ultimately, these devices should be portable and disposable. To this end, microfabricated capillary array electrophoresis (μCAE) devices are a promising candidate. μCAE can be used to carry out a wide variety of analyses such as amino acid analysis, protein and small analyte analysis, DNA fragment sizing and DNA sequencing with much greater efficiency than conventional methods (Micro Total Analysis Systems 2001 Ed. Ramsey & van den Berg, Kluwer Academic Press Dordrecht, 2001). Capillary electrophoresis (CE) measurements have been multiplexed to perform massively parallel, high-throughput genotyping on from 96 to 384—lane μCAE devices.
Most μCAE devices, however, still use conventional off-chip laser-induced fluorescence detection, including photomultiplier tubes, CCDs, optical filters, lenses, lasers, and so forth. Such a bulky detection system inhibits many potential point-of-analysis benefits possible with μCAE devices. In order to make a portable device, miniaturized excitation and detection systems must be developed. One means of accomplishing this is to use electrochemical detection, as addressed by Woolley et al. in Analytical Chemistry, 70, 684-698 (1998). On-chip electrochemical detection has attractive features such as a simple structure and ease of manufacturing, however, electrochemical detection has limited detection sensitivity compared to fluorescence. Furthermore, it is difficult to perform multiplexed assays such as DNA sequencing or certain protocols for single nucleotide polymorphisms (SNP) detection.
Fluorescent detection is very sensitive, especially when combined with laser excitation. Multiplexed detection is also feasible and routinely used in modern DNA sequencers. Therefore, it is beneficial to maintain fluorescence detection for bioanalytical chips and devise ways to miniaturize and integrate the excitation and detection system. Mastrangelo and co-workers have presented a system where they have fabricated silicon (Si) photodiodes directly on an Si wafer containing the microfluidic system (M. A. Burns et al., Science, 282, 484 (1998), J. R. Webster et al., Analytical Chemistry, 73, 1622 (2001)). This monolithic fabrication complicates electrophoresis because of the conductivity of Si substrates. In this case, the fluidic CE channel had to be electrically insulated from the silicon substrate by depositing material such as parylene-C, SiN, or SiO2 on the channel. The costs associated with the fabrication of single crystalline silicon systems are relatively high because of the high cost of single crystalline Si wafers and the multiple process steps that are required. Mariella has presented a portable DNA analyzer which performs the real-time polymerase-chain-reaction assays based on fluorescence detection using a homogenous TAQ-man assay, but his system is not designed for microfluidic channels and employs large microliter (μL to mL) volume of samples (Mariella, Jr., JALA, 6, 54 (2001)).